Amines - Overview, Examples, Types, Properties, FAQs

Types of Amines

Amines are divided from natural to aliphatic Amines and sweet Amines. In addition to Amines, aryl and alkyl are also divided into three categories based on the calculation of nitrogen atoms associated with nitrogen and the number of hydrogen atoms replaced.

1) Primary Amines and their examples

Primary amines are those in which one hydrogen atom of ammonia $\left(\mathrm{NH}_3\right)$ is replaced by an alkyl or aryl group. Their general formula is $\mathrm{R}-\mathrm{NH}_2$, where R represents an alkyl or aryl group.

Examples:

• Methylamine $\left(\mathrm{CH}_3 \mathrm{NH}_2\right)$ - simplest aliphatic primary amine
• Ethylamine $\left(\mathrm{C}_2 \mathrm{H}_5 \mathrm{NH}_2\right)$ - commonly used in organic synthesis
• Aniline $\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2\right)$ - simplest aromatic primary amine

2) Secondary Amines (2° Amines)

Secondary amines are formed when two hydrogen atoms of ammonia $\left(\mathrm{NH}_3\right)$ are replaced by alkyl or aryl groups. Their general formula is $\mathrm{R}_2 \mathrm{NH}$, where both R groups can be the same or different.

Examples:

• Dimethylamine $\left(\mathrm{CH}_3 \mathrm{NHCH}_3\right)$ - both groups are methyl
• Diethylamine $\left(\mathrm{C}_2 \mathrm{H}_5 \mathrm{NHC}_2 \mathrm{H}_5\right)$ - both groups are ethyl
• Ethylmethylamine $\left(\mathrm{C}_2 \mathrm{H}_5 \mathrm{NHCH}_3\right)$ - one ethyl and one methyl group

Secondary amines are widely used in the preparation of drugs, rubber chemicals, and corrosion inhibitors due to their strong basic and nucleophilic nature.

3) Tertiary Amines (3° Amines)

Tertiary amines are formed when all three hydrogen atoms of ammonia $\left(\mathrm{NH}_3\right)$ are replaced by alkyl or aryl groups. Their general formula is $\mathrm{R}_3 \mathrm{~N}$, where the three R groups may be the same or different.

Examples:

• Trimethylamine $\left(\mathrm{N}\left(\mathrm{CH}_3\right)_3\right)$ - all three groups are methyl
• Triethylamine $\left(\mathrm{N}\left(\mathrm{C}_2 \mathrm{H}_5\right)_3\right)$ - all three groups are ethyl
• Dimethylaniline $\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{~N}\left(\mathrm{CH}_3\right)_2\right)$ - one aryl and two methyl groups

Tertiary amines are less reactive toward acylation and alkylation but play an important role as catalysts and intermediates in the manufacture of dyes, pharmaceuticals, and surfactants.

4) Quaternary Amines (4°Amines)

Quaternary amines, also known as quaternary ammonium salts, are formed when all four hydrogen atoms of ammonia ( $\mathrm{NH}_3$ ) are replaced by alkyl or aryl groups, resulting in a positively charged nitrogen atom. Their general formula is $\mathrm{R}_4 \mathrm{~N}^{+} \mathrm{X}^{-}$, where $\mathrm{X}^{-}$is a counter anion such as $\mathrm{Cl}^{-}, \mathrm{Br}^{-}$, or $\mathrm{OH}^{-}$.

Examples:

• Tetramethylammonium chloride $\left(\mathrm{CH}_3\right)_4 \mathrm{~N}^{+} \mathrm{Cl}^{-}$
• Tetraethylammonium bromide $\left(\mathrm{C}_2 \mathrm{H}_5\right)_4 \mathrm{~N}^{+} \mathrm{Br}-$
• Cetyltrimethylammonium bromide (CTAB) - a common surfactant used in detergents

Quaternary ammonium compounds are ionic, water-soluble, and widely used as disinfectants, surfactants, and phase-transfer catalysts in organic synthesis.

Nomenclature of Amines

Amines are named according to the IUPAC system, which is based on the parent hydrocarbon chain attached to the nitrogen atom.

1. Simple (Aliphatic) Amines

• The longest carbon chain attached to the nitrogen is chosen as the parent chain.
• The suffix "-amine" is added to the name of the parent alkane.
• The position of the - $\mathrm{NH}_2$ group is indicated by a number (if needed).

Examples:

• $\mathrm{CH}_3 \mathrm{NH}_2 \rightarrow$ Methanamine (Common name: Methylamine)
• $\mathrm{C}_2 \mathrm{H}_5 \mathrm{NH}_2 \rightarrow$ Ethanamine (Common name: Ethylamine)
• $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{NH}_2 \rightarrow$ Propan-1-amine

2. Aromatic Amines

• When the –NH₂ group is directly attached to an aromatic ring, the compound is named as an aryl amine.
• The simplest aromatic amine is aniline $\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2\right)$.

Examples:

• $\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 \rightarrow$ Benzenamine (Common name: Aniline)
• $\mathrm{C}_6 \mathrm{H}_5 \mathrm{~N}\left(\mathrm{CH}_3\right)_2 \rightarrow$ N,N-dimethylbenzenamine (Common name: Dimethylaniline)

Properties of Amines

• Low amphibious Amines have a fishy-like odor.
• Basic Amines are a liquid at room temperature that contains three to four atoms in carbon, while the higher Amines are solid.
• Aniline and other aryl Amines are usually colorless. However, when stored in an open area, they are painted due to atmospheric oxidation.
• Low-density Amines aphids dissolve in water as they form bonding with the hydrogen atom of water. However, as the fraction of the hydrophobic alkyl component increases, the weight of the Amines molar decreases and the solubility also decreases.
• Ground Amines do not melt. Organic solvents such as ethanol, benzene, and ether are used to dissolve Amines easily.
• Primary and secondary Amines are often involved in the interaction of anime molecules because of the interaction of hydrogen with nitrogen that interacts with the hydrogen of other atoms.
• Intermolecular strength is seen more in the basic Amines compared to the secondary due to the presence of two hydrogen atoms. In contrast, high Amines have no intermolecular potential due to the lack of free hydrogen atoms.
• Aromatic Amines are relatively reduced than aliphatic Amines because two pairs of nitrogen atoms are formed and are less available with acid.

NCERT Chemistry Notes

Some Solved Examples

Question 1: Which of the following statement is true regarding the Hinsberg Test for Amines?
1) (correct) Methyl amine forms soluble compound with Hinsberg reagent and KOH
2) Dimethyl amine forms an insoluble compound with Benzene sulphonyl chloride and KOH
3) Trimethyl amine forms a soluble compound with Hinsberg reagent and KOH
4) The reagent used in Hinsberg's Test is Benzoyl Chloride

Solution:

Hinsberg's test is carried out with Benzene Sulphonyl Chloride and KOH.

$\begin{aligned}
& \mathrm{RNH}_2 \xrightarrow{\mathrm{PhSO}_2 \mathrm{Cl}} \text { Insoluble } \xrightarrow{\mathrm{KOH}} \text { Soluble } \\
& \mathrm{R}_2 \mathrm{NH}^{\mathrm{PhSO}_2 \mathrm{Cl}} \text { Insoluble } \xrightarrow{\mathrm{KOH}} \text { Insoluble } \\
& \mathrm{R}_3 \mathrm{~N} \xrightarrow{\mathrm{PhSO}_2 \mathrm{Cl}} \text { no reaction }
\end{aligned}$
Hence, the answer is the option (1)

Question 2: The presence of Nitro group in nitrobenzene is as certained by
1) Schiff's test
2) (correct) Mulliken and Barkar's Test
3) Both the above
4) None of the above

Solution:

As we learnt

Mulliken - Barker test -

Ethanolic solution of nitrobenzene is treated with zinc dust and $\mathrm{CaCl}_2$ solution. The mixture is heated and filter in a test tube containing Tollen's reagent a grey or black precipitate ( Ag mirror ) is formed.
wherein
$\mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}_2+\mathrm{Zn}+\mathrm{NH}_4 \mathrm{Cl} \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}-\mathrm{OH}$
$\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}-\mathrm{OH} \rightarrow \mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}+$ Silver mirror

Hence, the answer is the option (2).

Question 3: $\mathrm{CH}_3-\mathrm{X}+{ }^{\prime} \mathrm{X}^{\prime} \rightarrow \mathrm{CH}_3-\mathrm{CN}$
' $X$ ' is :-
1) (correct) KCN(alc)
2) CH3CN
3) CuCN
4) None of these

Solution:

As we learnt

Method of preparation of cyanide from alkyl halide -
By heating an alkyl halide with potassium cyanide in aqueous ethanolic solution.
-wherein

$R X+K C N \rightarrow R C N+K X$
Hence, the answer is the option (1).

Question 4: Which is the strongest base in aqueous solution?
A. Aniline $\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2\right)$
B. Ammonia $\left(\mathrm{NH}_3\right)$
C. Methylamine $\left(\mathrm{CH}_3 \mathrm{NH}_2\right)$
D. Nitrogen gas $\left(\mathrm{N}_2\right)$

Solution:

Basicity in water depends on how readily the lone pair on N accepts $\mathrm{H}^{+}$. Electron-donating alkyl groups increase electron density on $\mathrm{N} \rightarrow$ stronger base. Methylamine $>$ ammonia. Aniline's lone pair is delocalized into the benzene ring (resonance), so it is much weaker. ( $N_2$ is not a base here.)

Hence, the answer is the option (C).

Question 5: Give the IUPAC name of $\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{NH}-\mathrm{CH}_3$.
A. Ethylmethylamine
B. N -methylpropanamine
C. N -methylethanamine
D. 1-Aminoethane

Solution :

Parent chain is ethane → ethanamine. The methyl on N is indicated by N-methyl → N-methylethanamine (commonly called ethylmethylamine as trivial name).

Hence, the answer is the option (C).

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